Crystallization-Induced Diastereomeric Transformation of N-2'-t-Butyl-6'-iodobenzoyl-3-bromocarbazole

We previously reported the atropisomeric properties of 2'-t-butyl-6'-iodo-substituted N-benzoyl-3-bromocarbazole, i.e., the steric or electronic effects of the substituents restricted the rotation about the N-C7' and C7'-C1' bonds to separate four stereoisomers (cis/trans fo...

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Published inChemical and Pharmaceutical Bulletin Vol. 62; no. 8; pp. 836 - 838
Main Authors Susumu Kayama, Norihiko Tani, Yuka Takahashi, Hidetsugu Tabata, Shintaro Wakamatsu, Tetsuta Oshitari, Hideaki Natsugari, Hideyo Takahashi
Format Journal Article
LanguageJapanese
Published Pharmaceutical Society of Japan 01.08.2014
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Summary:We previously reported the atropisomeric properties of 2'-t-butyl-6'-iodo-substituted N-benzoyl-3-bromocarbazole, i.e., the steric or electronic effects of the substituents restricted the rotation about the N-C7' and C7'-C1' bonds to separate four stereoisomers (cis/trans for the N-C7' axis, aR/aS for the C7'-C1' axis). Furthermore, the 2'-t-butyl-6'-iodo-substituted N-benzoyl 3-bromocarbazole was confirmed to be a gear molecule, in which the rotation about the C7'-C1' bond was in perfect concert with that about the N-C7' bond. Herein, we report a unique crystallization-induced diastereomeric transformation found in this molecule. In the isolation process, where the product is recrystallized from the diastereoisomeric mixture, in situ isomerization and selective crystallization could lead to a diastereomeric transformation, and a mixture of diastereomers (trans/cis=54:46) was converted to trans-isomer-enriched crystals (trans/cis>96:4) in 95% yield. Conformational analysis clarified the preference for the trans versus cis isomer.
ISSN:0009-2363